In the semiconductor industry, alignment equipment (aligners) is used to align substrates, in particular wafers, with one another, in order to join them together in a further process step. The joining process is called bonding.
Alignment processes in which alignment markings are present on the substrate surfaces to be bonded are referred to as face-to-face alignments.
Inasmuch as the substrates are not transparent for the electromagnetic radiation that is used to measure the substrates in order to enable a detection and/or determination of the position and orientation of the alignment markings from the external substrates surfaces facing away from the substrates surfaces to be bonded, alignment markings are detected with image detection means between the substrates before the substrates are brought towards one another.
This had various drawbacks, in particular particle contamination by the camera and a large distance between the substrates to allow the positioning of the camera between two substrates for the detection. Alignment errors during the mutual approach arise from this.
An improvement of the face-to-face alignment is represented by the alignment equipment of publication U.S. Pat. No. 6,214,692B1, which can be regarded as the closest prior art. In the case of the latter, two groups of lenses are used with in each case two lenses lying opposite each other, in order to create a system with two reference points in relation to which the substrates are mutually positioned. The reference points are points of intersection of the optical axes of two lenses lying opposite one another.
The problem with the prior art includes the fact that the increasing demands made on alignment accuracy can no longer be met by the disclosed open, controlled methods.
Publication U.S. Pat. No. 6,214,692B1 is based on the comparison and the position correction of two images of alignment markings. The position of alignment markings on the two substrates arranged face-to-face is detected individually by means of a camera system. A positioning table (substrate holder and stage) is actuated from a calculated relative orientation and relative position of the alignment markings in such a way that the incorrect position is corrected. The positioning table comprises guide transducers and incremental path transducers positioned close to the drive.